On-chip integration of MMIC and single photon detectors

ABSTRACT

A photon detection device and method of fabricating a photon detection device are provided. The photon detection device comprises a first input terminal for receiving a DC input voltage, a second input terminal for receiving an AC input voltage and a bias tee connected to the first and second input terminals and configured to combine the AC and DC input voltages to form a combined voltage on an output of the bias tee. A first single photon detector is connected to the output of the bias tee and configured to receive the combined voltage from the bias tee, register a detection signal based on only a single photon being incident on the first single photon detector and output the detection signal indicative of the detected photon. A first output terminal is connected to an output of the first single photon detector for outputting the detection signal. The photon detection device is formed in a single integrated circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. application Ser. No. 15/903,537 filedFeb. 23, 2018, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to photon detection devices and methodsof manufacturing photon detection devices.

BACKGROUND

Photon detectors are used in a number of applications includingindustrial inspection, environmental monitoring, testing of fibre opticcables and components, medical imaging, chemical analysis and scientificresearch.

Photon detectors are also important for many applications in quantuminformation technology, such as linear optics quantum computing, quantumrelays and repeaters, quantum cryptography, photon number stategeneration and conditioning, and characterisation of photon emissionstatistics of light sources.

There is a continuing need to improve the photon detectors used in theseapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

Arrangements of the present invention will be understood and appreciatedmore fully from the following detailed description, made by way ofexample only and taken in conjunction with drawings in which:

FIG. 1 shows an integrated circuit schematic for a single photondetector according to a first arrangement;

FIG. 2 shows an integrated circuit schematic for a single photondetector with a differential output for capacitive response cancellationaccording to an arrangement;

FIG. 3 shows a simulation of the voltage output for the circuit of FIG.1;

FIG. 4 shows a simulation of the voltage output at each APD and theoutput voltage of the subtractor from FIG. 2;

FIG. 5 shows a simulation of the output voltage of the subtractor fromFIG. 2 on a pico-volt (pV) scale;

FIG. 6A shows an on-chip resistor;

FIG. 6B shows two types of on-chip diffusion resistors;

FIG. 7A shows an on-chip capacitor;

FIG. 7B shows an alternative on-chip capacitor;

FIG. 7C shows a metallic spiral inductor suitable for on-chipimplementation;

FIG. 8 shows another inductor suitable for on-chip implementation;

FIG. 9 shows an example of physical on-chip layout according to onearrangement;

FIG. 10 shows an alternative arrangement to FIG. 9;

FIG. 11 shows initial steps in a fabrication sequence for producing asingle photon detector; and

FIG. 12 shows third and fourth steps in the fabrication sequence of FIG.11;

FIG. 13 shows a fifth step in the fabrication sequence of FIGS. 11 and12;

FIG. 14 shows a sixth step in the fabrication sequence of FIGS. 11-13;

FIG. 15 shows two final steps of the fabrication sequence of FIGS.11-14;

FIG. 16 shows cross-sectional views of a capacitor and an inductor; and

FIG. 17 shows a method for forming a single photon detector according toan arrangement.

DETAILED DESCRIPTION

According to a first arrangement there is provided a photon detectiondevice comprising a first input terminal for receiving a DC inputvoltage, a second input terminal for receiving an AC input voltage and abias tee connected to the first and second input terminals andconfigured to combine the AC and DC input voltages to form a combinedvoltage on an output of the bias tee. A first single photon detector isconnected to the output of the bias tee and configured to receive thecombined voltage from the bias tee, register a detection signal based ononly a single photon being incident on the first single photon detectorand output the detection signal indicative of the detected photon. Afirst output terminal is connected to an output of the first singlephoton detector for outputting the detection signal. The photondetection device is formed in a single integrated circuit.

By forming the photon detection device on a single chip, in a singleintegrated circuit, the size of the photon detection device can begreatly reduced compared to systems that have an external bias tee. Inaddition, by implementing the device in a single integrated circuit, thedistance between components and the size of components can be reduced toreduce the noise in the system and increase the speed. This isparticularly important in single photon detection systems where thedetection signals may be quite small compared to the capacitive responseof the single photon detector(s) and which can operate at GHzfrequencies.

A single photon detection device is able to detect light based on onlyone photon. This is useful in, for instance, quantum cryptography.

The output of the single photon detector may be measured over aresistor. The first single photon detector may be connected on a firstline between the output of the bias tee and ground, the first linecomprising the first single photon detector connected in series to aresistor, wherein the output terminal is connected between the firstsingle photon detector and the resistor. Connected to ground couldinclude being connected to a terminal configured to be connected toground, or being connected directly to ground.

Advantageously, the bias tee comprises a first input line connected tothe first input terminal, the first input line comprising a low passfilter, and a second input line connected to the second input terminal,the second input line comprising a high pass filter.

The bias tee is able to apply a low pass filter to the DC input voltageand a high pass filter to the AC input voltage to filter out unwantedDC/AC components. The bias tee can therefore combine clean AC and DCsignals to form the combined voltage provided to the first single photondetector. By implementing the high and low pass filters in theintegrated circuit with the single photon detector, the distance thatthe filtered signal has to travel is reduced, thereby reducing the noisethat may be introduced into the bias voltage provided to the firstsingle photon detector. In addition, reducing this distance and reducingthe size of the components reduces the capacitive response of thecircuit and therefore allows the circuit to be driven at a higher speed,increasing the maximum gating speed for the system.

Advantageously, the first and second input lines may combine at a node,and wherein a connection between the node and the first single photondetector is less than 200 μm long.

In one arrangement the low pass filter comprises an inductor and aresistor and/or the high pass filter comprises a capacitor. The inductorand resistor may be connected in series.

Advantageously, the low pass filter may further comprise a secondcapacitor connected in parallel to the inductor and resistor. The secondcapacitor may connect the first input terminal to ground. This may beused for additional low pass filtering.

According to an arrangement a capacitive line for impedance matching isconnected between the output of the bias tee and ground, in parallel tothe first single photon detector. This allows the impedance within thesystem to be balanced to maximize the power transfer or minimize signalreflection from the load. The capacitive line may comprise a thirdcapacitor connected in series to a second resistor.

The first single photon detector may be a single photon avalanchedetector.

According to a further arrangement the photon detection device furthercomprises a second single photon detector connected to the output of thebias tee, in parallel to the first single photon detector, andconfigured to receive the combined voltage from the bias tee and outputan output signal, and a second output terminal connected to an output ofthe second single photon detector for outputting the output signal.

This allows the capacitive response of one of second single photondetector to be used to cancel the capacitive response of the firstsingle photon detector to help to isolate signals from the detection ofsingle photons more effectively. Advantageously, the first and secondsingle photon detectors may be located adjacent to each other, forinstance, within 150 μm of each other, so that they experience similarelectric fields. This helps to ensure that the second single photondetector has a similar capacitive response to the first single photondetector.

Whilst the above discussion relates to canceling signals from the firstsingle photon detector using the second single photon detector, thereverse may apply, wherein the signals from the first single photondetector are subtracted from the signal from the second single photondetector.

The second single photon detector may be a single photon avalanchedetector.

Advantageously, the second single photon detector may be identical tothe first single photon detector. This helps to match the capacitiveresponses of the two single photon detectors, so that one may act as adummy single photon detector for noise cancellation.

Both the first and second single photon detectors may be single photonavalanche detectors. The two single photon detectors may have the samelayer structure and doping. Having said this, the second single photondetector may have a capping layer formed over it to block photons. Thisensures that only the capacitive response is output, and no signalscaused by photons are output, so that it always acts as a dummy singlephoton detector.

The first and second single photon detectors may have been formedsimultaneously. For instance, the two single photon detectors may havebeen formed sharing the same deposition and/or doping steps. This canhelp match the capacitive response of the two single photon detectors byreducing fabrication differences between the two.

Advantageously, the photon detection device may be further configured toblock photons incident on the second single photon detector. Thisensures that the output signal of the second photon detector does notinclude detection signals so that only the capacitive response isoutput. This may be in the form of a capping layer deposited over thesingle photon detector to block photons. Alternatively, a removable capmay be used (for instance, a slider or a cap that may be placed over thesecond single photon detector). The first single photon detector mayidentical to the second single photon detector beneath any cappinglayer/cap.

The first and second single photon detectors may be located within 150μm of each other. This can help reduce fabrication differences andensure similar local electric fields (that would affect the noise) areapplied to the two single photon detectors. Optionally, the first andsecond single photon detectors may be located within 50 μm or 20 μm ofeach other.

According to an arrangement there is provided a photon detection systemcomprising a photon detection device as described herein and a noisecancellation circuit configured to subtract the output signal from thedetection signal to remove noise from the detection signal. The noisecancellation circuit may be external to the integrated circuit (thephoton detection device) and connected via terminal(s) in the integratedcircuit. The noise cancellation circuit may be a self-differencingcircuit (where one single photon detector is used) or a subtractioncircuit (where two single photon detection circuits are used).

According to an arrangement the single photon detection system comprisesa photon detection device as described herein, or a photon detectionsystem as described above, and a single photon source configured todirect only one photon at a time to the first single photon detector,wherein the single photon detection system is configured to not directlight towards the second single photon detector. Accordingly, whilst thesystem or device may be configured to block light to one of the singlephoton detectors, in an alternative arrangement, both detectors areexposed and able to detect photons, but only one is utilised to detectlight at one time so that the other may be used to cancel out thecapacitive response.

According to a further arrangement there is provided a method forfabricating a photon detection device on a substrate, the methodcomprising: forming a first single photon detector on the substrate,wherein the first single photon detector is configured to register adetection signal based on only a single photon being incident on thefirst single photon detector and output the detection signal indicativeof the detected photon; forming a bias tee on the substrate, the biastee being configured to combine AC and DC input voltages to form acombined voltage on an output of the bias tee, the output of the biastee being connected to the first single photon detector to provide thecombined voltage to the first single photon detector; forming on thesubstrate a first input terminal for receiving the DC input voltage, thefirst input terminal being connected to the bias tee to provide the DCinput voltage to the bias tee; forming on the substrate a second inputterminal for receiving the AC input voltage, the second input terminalbeing connected to the bias tee to provide the AC input voltage to thebias tee; and forming on the substrate a first output terminal connectedto an output of the first single photon detector for outputting thedetection signal, wherein the photon detection device is formed in asingle integrated circuit.

Forming the photon detection device on a single substrate in a singleintegrated circuit reduces the size of the system, reduces the noise inthe system and allows the system to operate at a faster speed.

The method may further comprise forming on the substrate a second singlephoton detector connected to the output of the bias tee, in parallel tothe first single photon detector, and configured to receive the combinedvoltage from the bias tee and output an output signal and forming on thesubstrate a second output terminal connected to an output of the secondsingle photon detector for outputting the output signal.

The second single photon detector may act as a dummy single photondetector to cancel out the capacitive response from the first singlephoton detector.

The first and second single photon detectors may be identical. In onearrangement the first and second single photon detectors are formedsimultaneously. This allows the same materials and dopants to beutilised and reduces the fabrication differences between the twodetectors.

According to a further arrangement the first and second single photondetectors are formed within 150 μm of each other.

Single photon avalanche detectors (SPADs) are highly sensitivephotodetectors that are able to detect individual photons. SPADs utiliseavalanche breakdown to amplify the current caused by individual photonshitting the detector.

As SPADs need to detect very small signals, a high degree of precisionand noise cancellation is required. Measurement error can be reduced bygating the SPAD, where the SPAD is only turned on during the time that aphoton is expected to be received. Having said this, the speed of gatingcan be limited by the speed and precision of signals traveling throughthe system. In addition SPADs systems can be quite large.

There is therefore a need for a faster and more precise single photondetector that can be implemented in a smaller system.

Arrangements described herein incorporate driving circuitry onto thesame chip as single photon detector(s). This reduces the distance thatsignals need to travel, thereby reducing error and increasing the speedin the system.

FIG. 1 shows an integrated circuit schematic for a single photondetector according to a first arrangement. The components shown areimplemented on as an integrated circuit on a single chip.

Two input signals are received. A direct current (DC) signal is receivedat a DC input 102 and an alternating current (AC) signal is received atan AC input 104. The two signals are combined at a bias tee 106 whichsupplies the combined signal to an input of the avalanche photondetector (APD) 108. The bias voltage applied to the APD thus comprisesboth a DC component and an AC component.

The bias voltage applied to the APD is above the breakdown voltage atits highest values and below the breakdown voltage at its lowest values.When the bias voltage exceeds the breakdown voltage the detector isgated “on” and is able to detect individual photons incident on adetection region of the APD 108. When the voltage is below the breakdownvoltage the detector is gated “off” and the detector is insensitive tophotons. The frequency of the AC voltage component is thus the gatingfrequency. The gating frequency may be synchronised with the drivingfrequency of the photon source in a QKD system for example.

Depending on the operation temperature and the device structure, thebreakdown voltage for APDs can vary from 20 to 300 V.

The AC signal is a voltage that oscillates at the gating frequency ofthe device. The DC signal is a steady bias voltage that is adjustablebelow or above the breakdown voltage of the APD 108.

The DC input 102 and AC input 104 are connected to first and second armsof the bias tee 106 respectively. The bias tee circuit 106 is connectedto a cathode contact of the APD 108 (however, the APD may be reversed,so that the bias tee circuit 106 is connected to the anode, providedthat the bias voltage is reversed). The bias tee 106 comprises, on afirst arm of the tee, an inductor L1 and resistor R1 connected in seriesto the DC input 102, and on a second arm of the tee, a first capacitorC1 connected to the AC input 104. The inductor L1 and the first resistorR1 act as a low pass filter for the DC signal.

The first arm further comprises a second capacitor C2 connecting the DCinput 102 to ground. The second capacitor works to provide additionallow-pass filtering to the first arm of the bias tee 106.

In an alternative arrangement, the second capacitor C2 is not utilised.Instead, the inductor L1 and first resistor R1 can act as a low passfilter without being connected in parallel to the second capacitor C2.

The second arm of the bias tee comprises a first capacitor C1. The firstcapacitor C1 acts as a high pass filter for the AC signal. The first andsecond arms join at a node 107 (to combine the filtered AC and DCinputs) and continue on via an output line of the bias tee 106.

A third capacitor C3 is connected to the output of the bias tee 106 inparallel to the APD 108. The third capacitor C3 is connected to groundvia a second resistor R2. This allows the circuit to be tuned forimpedance matching.

An output of the APD 108 is also connected to ground via a thirdresistor R3. The output of the APD 108 is connected to an outputterminal 110 of the circuit. This provides an output voltage (V_(out))that can be filtered to remove noise to allow signals from individualphotons to be detected. For instance, the output voltage V_(out) may beinput into a self-differencing circuit to cancel the capacitive responseof the APD 108 and to leave only the avalanche signal arising fromphoton detection. In a self-differencing mode of operation, thebackground of the output signal is removed by comparing a part of thesignal with an earlier part of the signal that is phase shifted by 180degrees.

In a further arrangement, the (filtered) output voltage is input into adiscriminator configured to compare the measurement of the avalancheevent with a predetermined level. This provides a digital output when anavalanche event is detected.

The circuit of FIG. 1 is implemented on a single chip (in a singleintegrated circuit). This reduces the size of the detection system ingeneral and reduces the distance that the input signals need to travelto drive the APD 108. This therefore reduces the amount of noise in thesystem (e.g. by reducing inductive and capacitive coupling in thedriving circuit) compared to systems with external driving circuitry.This is in contrast to external driving circuitry which would have to beconnected to the APD via long wires (e.g. more than 1 m long) which canbe subject to a relatively large amount of noise.

In addition, reducing the distance that the signals need to travelallows the circuit to function more quickly by reducing the amount ofparasitic inductance and capacitance within the system. The APD cantherefore be driven at a faster clock frequency. This means that thesystem can achieve gating at frequencies higher than 2 GHz.

The capacitance, resistance and inductance of the various components ofthe circuit may be adapted to tune the system. For instance, whilst FIG.1 states that the second R2 and third R3 resisters have resistances of50Ω each, alternative resistances may be used based on the system athand provided that R2 and R3 have equal resistances for impedancematching.

To help further reduce background response, a second (“dummy”) APD maybe used. The dummy APD is not used to detect light, but simply to act asa basis for canceling noise from the output signal of the first APD 108.

FIG. 2 shows an integrated circuit schematic for a single photondetector with a differential output for capacitive response cancellationaccording to an arrangement.

The circuit is similar to that of FIG. 1; however, a dummy APD (APD1)218 is utilised in parallel to a first APD (APD) 208. The first APD 208acts to detect photons in the same manner as APD 108. The dummy APD 218does not detect photons. This may be because no photons are shone on thedummy APD 218, or may be due to the dummy APD 218 being covered toprevent light reaching its detection region.

The input of the dummy APD 218 is connected to the output of a bias tee(similar to bias tee 106), in parallel to the first APD 208. The outputof the dummy APD 218 is connected to ground via a fourth resistor R4 ina similar arrangement to the first APD 208 and the third resistor R3.

The outputs of the dummy APD 218 and the first APD 208 may be connectedto a differential amplifier 220. This subtracts the signal from thedummy APD 218 from the signal from the first APD 208 to remove thebackground response of the first APD 208 from the signal and isolateavalanche pulses caused by detected photons.

The differential amplifier 220 comprises an operational amplifier 230.The operational amplifier 230 comprises an inverting input (labeled “−”)and a non-inverting input (labeled “+”). An output of the operationalamplifier 230 is connected to the inverting input via a fifth resistorR5. The output of the dummy APD 218 is connected, via a sixth resistorR6, to the inverting input. The output of the first APD 208 isconnected, via a seventh resistor R7, to the non-inverting input. Thenon-inverting input is connected to ground via an eighth resistor, R8.The output of the operational amplifier 230 is connected to ground via aninth resistor R9. The output of the operational amplifier is alsoconnected to an output node PR2 for outputting the noise canceled signal(e.g. to a comparator to provide a digital output).

The resistor values of the arrangement of FIG. 2 are detailed inTable 1. Having said this, alternative resistances may be used,depending on the characteristics of the system.

TABLE 1 resistance values for the arrangement of FIG. 2. ResistorResistance R1 100 Ω R2  50 Ω R3  50 Ω R4  50 Ω R5 100 Ω R6 100 Ω R7 100Ω R8 100 Ω R9   1 kΩ

The first APD 208 and the dummy APD 218 are substantially identical.This means that the capacitive response of the dummy APD 218 can beconsidered substantially the same as the capacitive response of thefirst APD 208. This is particularly the case if the dummy APD 218 islocated in close proximity to the first APD 208 (for instance, within 20μm). This means that the dummy APD 218 and the first APD 208 will besubject to similar electrical fields, thereby having similar capacitiveresponses.

The dummy APD 218 is identical to the first APD 208 in that it is of thesame construction (e.g. having the same doping levels and layerthicknesses). This means that the dummy APD 218 and the first APD 208have the same breakdown voltage. The resistance of the third R3 andfourth R4 resistors is the same (in this case, 50Ω) and the first APD208 and the dummy APD 218 receive the same bias voltage from the biastee. This means that the two APDs will have substantially the sameelectrical response and output substantially the same signals with theexception that the output of the first APD 208 includes voltage spikescaused by the detection of individual photons.

As the dummy APD 218 and the first APD 208 are located on the same chipand at close proximity, there will be fewer fabrication differences thanif the APDs were fabricated separately. This is because, by fabricatingthe APDs on the same chip and at close proximity, the crystal structureof the layers of the two APDs will likely be identical. In addition, thetwo APDs can be fabricated at the same time, sharing the same doping anddeposition steps. The doping of the layers will therefore also likely beidentical. Accordingly, by fabricating the two APDs on the same chip, atclose proximity and at the same time, the dummy APD 218 will be a bettersurrogate for the first APD 208 and will therefore provide improvedbackground cancellation.

In addition, by keeping the APDs on the same chip, the distance betweenthe APDs can be reduced, thereby reducing the noise caused by signalstraveling long distances, and reducing the difference in phase betweenthe signals output from the two APDs (thereby improving the backgroundcancellation when the two signals are subtracted).

The differential amplifier 220 acts as a subtractor. There is no need toimplement the differential amplifier 220 on the chip. Instead, thedifferential amplifier 220 may be a circuit external to the circuit onthe chip. The differential amplifier 220 may be connected to the on-chipcircuit via output terminals connected to the outputs of the two APDs,similar to the output terminal 110 in FIG. 1. Equally, the sectionslabeled “GND” in FIGS. 1 and 2 need not be always connected to ground,but could simply represent terminals that can be connected to groundwhen the device is in use.

FIG. 3 shows a simulation of the voltage output for the circuit ofFIG. 1. The noise caused by the capacitive response of the system may beremoved using a self-differencing circuit.

FIG. 4 shows a simulation of the voltage output at each APD and theoutput voltage of the subtractor from FIG. 2. The output of the firstAPD 208 is depicted via a smooth line, the output of the dummy APD 218is depicted via a series of triangles and the output of the differentialamplifier 220 (the subtractor) is depicted via a dotted line with hollowtriangles. In the present arrangement, neither APD is detecting photons.Accordingly, the signals relate solely to the background noise of thesystem.

FIG. 5 shows a simulation of the output voltage of the subtractor fromFIG. 2 on a pico-volt (pV) scale. The outputs of the two APDs aresubstantially the same. Accordingly, when the output of the dummy APD218 is subtracted from the output of the first APD 208, most of thevoltage fluctuations are removed. This effectively cancels out thecapacitive response of the first APD, and brings the background noisedown to the region of a few pico-volts. The reduced noise allows photondetection signals to be detected more effectively. This is particularlythe case given that the avalanche signal produced by a detected photonis generally substantially smaller than the capacitive output of theAPDs.

FIG. 6A shows an on-chip resistor. The resistor comprises a thin filmconductor 606 (for instance, NiCr or TaN) deposited between two metalcontacts 602, 604. The resistance of the resistor is determined by thelength (l), width (w), thickness (t) and type of metal used for the thinfilm:

$R = \frac{R_{sh}l}{w}$

wherein R_(sh) is the sheet resistance for the given conductor. Thesheet resistance is determined by the bulk resistivity (ρ) of theconductor and the thickness (t) of the film:

$R_{s} = \frac{\rho}{t}$

For instance, the sheet resistance for a NiCr film of 34 nm is 50 Ω/sq.

As the resistance increases with the length of the thin film, the thinfilm may be formed into a snaking “zig-zag” pattern to increase thelength whilst keeping the cross-section of the device small.

FIG. 6B shows two types of on-chip diffused resistors. An n-typediffused resistor is shown above a p-type diffused resistor.

The n-type diffused resistor comprises two metal contacts 612, 614deposited on an n+ doped region 616 of an n doped InP cap layer (N-cap).The two metal contacts 612, 614 are spaced apart from each other on thesurface of the n+ doped region 616. An insulator (for instance, SiN) isdeposited over the n+ doped region 616, between the metal contacts 612,614. The resistance of a diffused resistor can be calculated in the samemanner as for a thin film resistor, utilising the sheet resistance(R_(s)), length (l) and width (w) of the n+ doped region.

The p-well resistor is formed in the same manner as the n-type diffusedresistor; however, a p-well is used, rather than an n+ doped region, andp+ doped regions are located below the metal contacts. The p+ dopedregions have a higher doping concentration than the p-well.

FIG. 7A shows an on-chip capacitor. This on-chip capacitor utilises ametal plate 702 separated from a n+ or p+ doped semiconductor region 704by an insulator (dielectric) 706. The insulator may be a nitride.

FIG. 7B shows an alternative on-chip capacitor. This capacitor issimilar to the capacitor of FIG. 5A; however, the doped semiconductorregion is replaced with a second metal plate embedded within theinsulator.

In the arrangement of FIG. 5B the capacitance (C) is:

$C = {ɛ_{0}ɛ_{s}\frac{lw}{d}}$

wherein l and w are the length and width of the metal platesrespectively, d is the separation of the metal plates, ε₀ is theelectric constant and ε_(s) is the relative static permittivity of thedielectric between the metal plates.

For a capacitor with a nitride layer of 50 nm separating two square 40μm×40 μm metal plates, the capacitance is 1 pF.

FIG. 7C shows a metallic spiral inductor suitable for on-chipimplementation. The inductor comprises a metallic coil spiraling inwardsupon itself on the surface of a substrate. At the centre, a via allowsthe inner end of the inductor to pass to a second level below the spiralto exit the spiral without forming a short circuit. The inductance isdependent on the width, spacing, inner diameter, outer diameter andnumber of turns in the spiral.

FIG. 8 shows another inductor suitable for on-chip implementation. Inthis arrangement, two conductor paths spiral inward from two contactpoints. The paths cross each other each half turn, moving inwards by onestep. The paths can cross if one passes over the other, with aninsulator separating the two paths. At the centre of the spiral the twopaths join. The inductor is a spiral with six inward steps (N=6) andwith a diameter (D) of 266 μm. This provides an inductance of 8 nH.

FIG. 9 shows an example of physical on-chip layout according to onearrangement. The on-chip layout is similar to the arrangement of FIG. 2.

The circuit comprises a DC input terminal 902 and an AC input terminal904. The DC input terminal 902 is connected to a first arm of a biastee. The AC input terminal 904 is connected to a second arm of the biastee. The first and second arms join at a node 910 which is connected toan output of the bias tee.

The first arm of the bias tee comprises an inductor 906 connected inseries to a first resistor 908. The inductor 906 is connected to thenode 910 whilst the first resistor 908 is connected to the DC inputterminal 902. The second arm of the bias tee 910 comprises a firstcapacitor 905 connected between the AC input terminal 904 and the node910.

The output of the bias tee is connected to three parallel paths. Thefirst parallel path comprises a second resistor 912 connected to a firstground terminal 916 (GND) via a second capacitor 914. The second pathcomprises a first APD 922 connected to a second ground terminal 926(GND) via a third resistor 924. The third parallel path comprises asecond APD 932 connected to a third ground terminal 936 (GND) via afourth resistor 934.

Portions of the connections between the APds 922, 932 and the third 924and fourth 934 resistors are hidden in FIG. 9 as they occur through alower layer. Each APD 922, 932 has a respective via 923, 933 that passesthrough to a lower layer which provides a connection to the cathode ofthe respective APD. In this arrangement the bias voltage is provided bythe bias tee to the anode of each APD 922, 932 and the output of eachAPD 922, 932 is taken from the cathode of each APD 922, 932. It will beappreciated that this arrangement can be reversed, if the bias voltageis reversed. Accordingly, as shall be discussed later, the bias voltagemay be applied to the cathodes and the outputs may be taken from theanodes.

The output of the second APD 932 is also connected to a first outputterminal 940 (Out1). The output of the first APD 922 is also connectedto a second output terminal 942 (Out2)

One of the two APDs may function as a dummy APD to cancel out noise fromsignals detected in the other APD. In one arrangement, both APDs areable to detect photons, and one of the APDs is used as a dummy APDsimply by not having light shone upon its detection region. In analternative arrangement, only one of the APDs is configured to detectlight, whilst the other is not configured to detect light. For instance,the dummy APD may have a light blocking layer placed over its detectionregion to prevent photons from reaching the detection region. Other thanthis, the two APDs are identical.

All of the components in FIG. 9 are deposited on a single chip in asingle integrated circuit. The DC input 902 is configured to receive aDC input voltage. The AC input 904 is configured to receive an AC inputvoltage. The three ground terminals 916, 926, 936 are configured to beconnected to ground. The first 940 and second 942 output terminals areconfigured to be connected to a subtractor circuit such as the one shownin FIG. 2.

The capacitors, resistors and inductor of FIG. 9 may be any of thoseshown in FIGS. 6A-8, or any alternative arrangements.

FIG. 10 shows an alternative arrangement to FIG. 9. This arrangement issimilar to that of FIG. 9; however, the APds 1022, 1032 are biased inreverse relative to the arrangement of FIG. 9. The output of each APD1022, 1032 is taken from the anode of each APD 1022, 1032. The cathodeof each APD 1022, 1032 is connected to the output of the bias tee. Asthe cathodes are located at a lower level, a vias 1035 is provided toconnect the bias tee to the APds 1022, 1032.

The arrangements of FIGS. 9 and 10 are drawn to scale. By fabricatingthe two APDs on the same chip, the two APDs can be formed simultaneouslyusing the same doping and deposition steps.

Forming the bias tee and APDs on the same chip can help to greatlyreduce the size of the photon detector and can help reduce noise in thesystem.

In FIG. 10, the first APD 922 is around 80 μm away from the node 910 ofthe bias tee. This means that the combined, filtered bias voltage doesnot travel far to the APDs, therefore reducing the amount of noiseintroduced into the bias voltage.

In the arrangement of FIG. 10 the APDs are located around 35 μm awayfrom each other. Forming the APDs in proximity to each other reduces thefabrication differences between the two APDs

FIG. 11 shows initial steps in a fabrication sequence for producing asingle photon detector. A schematic cross-sectional view the initiallayers for producing the APDs in FIG. 10 is shown. The photon detectiondevice may be fabricated using integrated circuit processing.

Two APDs are formed side-by-side. The cross-sectional view is takenalong a plane passing horizontally across FIG. 10, in line with the via1035.

The basis for the heterostructure is a substrate 1, on which thesubsequent layer structure is fabricated. The substrate may be an InPsubstrate for example.

A uniform heterolayer, the second layer 2, is deposited on saidsubstrate 1. The second layer 2 may be an un-doped or lightly dopedn-type InGaAs layer for example.

A uniform n+ type heterolayer, the highly doped layer 4, is deposited onsaid second layer 2. This layer may be a highly doped n-type InP layerfor example.

A uniform layer, the first layer 3 is deposited on said highly dopedlayer 4. The first layer 3 may be un-doped or lightly doped n-type InPfor example.

A cross-sectional view of the device at this stage in fabrication isshown in i.

Areas of highly-doped p-type material 5 are incorporated into the firstlayer 3. The areas may be incorporated by Zn diffusion, or alternativelyby gas immersion laser doping or ion implantation for example.

In an arrangement, further areas of highly doped material, forming theguard ring regions 6, are also incorporated into the first layer 3. Theguard ring regions may be formed in the same step as the highly dopedregions 5, or in a separate step, and by the same method or by adifferent method.

A cross-sectional view of the device at this stage in fabrication isshown in ii.

In an alternative arrangement, the first 3 and second 2 layers may besilicon, in which p-type and n-type doping may be achieved using Boronor Phosphorous impurities respectively. The device may alternatively bebased on a Silicon-Germanium heterostructure or based on any of theIII-V class of semiconductors.

In an alternative arrangement, the device comprises highly n-dopedregions 5 which are incorporated into a moderately doped n-typeheterolayer 3, for example by gas immersion laser doping, implantationor diffusion.

In general, each APD comprises a first layer 3 of a first conductivitytype and a second layer 2 of the first conductivity type. These arelightly doped or even un-doped layers. The APD further comprises ahighly doped layer 4 of the first conductivity type. The highly dopedlayer 4 is overlying and in contact with the second layer 2. The firstlayer 3 is overlying and in contact with the highly doped layer 4.

The first conductivity type is n-type and the second conductivity typeis p-type. However, it will be appreciated that using alternativestructures the first conductivity type can be p-type and the secondconductivity type n-type.

The second layer 2 is overlying and in contact with a substrate 1.Alternatively, an intermediate layer or layers, such as a buffer layer,may be provided.

The first layer 3 comprises a highly doped region 5 of the secondconductivity type for each APD. The highly doped regions 5 have a higherdopant concentration than the remainder of the first layer 3. Theseregions are “islands”, i.e. each is laterally separated from the otherhigh dopant concentration region(s) 5.

For the avoidance of doubt, the term “high dopant concentration region”refers to the concentration of the carriers donated by the dopant.

The highly doped regions 5 are located at the surface of the first layer3. The depth of the highly doped regions is less than the depth of thefirst semiconductor layer 3.

The highly doped regions 5 have a circular shape seen in the plan view,in other words they have a cylindrical shape. The highly doped regionscan in principle be any shape however, including polygonal and rounded.In this case, the highly doped regions have a portion having a smallerdiameter and a portion having a larger diameter at the surface of thestructure. This reduces edge breakdown.

The structure may also comprise guard rings 6. The guard rings 6 arearranged around the outside of the highly doped regions 5. In this case,the guard rings 6 are circular, however, they can in principle be anyshape, including polygonal and rounded, with the overall geometry beingmatched to the shape of the highly doped regions 5. The guard rings 6are also highly doped regions of the first layer 3 and are highly dopedregions of the second conductivity type. The guard rings 6 have the sameconductivity type as the highly doped regions 5. The guard rings 6 arelocated at the surface of the first layer 3. The depth of the guardrings 6 is less than the depth of the first semiconductor layer 3.

As shall be discussed later, a separate anode contact is connected toeach metal contact region 8, such that each metal contact region, andthus each detection region, is connected to a separate anode. A singlesecond metal contact region 10 is formed on the opposite side of thesubstrate 1 and is connected to a cathode.

In use, a p-i-n junction is formed from the highly doped p-type region5, the n-type layer 3 and the highly doped n-type layer 4, forming anavalanche region.

A voltage is applied between each anode and the cathode. A high electricfield is generated across the interface between the highly doped n-typelayer 4 and each highly doped p-type layer 5. Avalanche multiplicationcan occur in this region when a suitable bias is applied across thejunction.

The depth of the highly-doped layer 4 can be less than 0.1 μm such thata thin junction with a shallow depletion region is achieved, with theAPD having a corresponding low breakdown voltage. The breakdown voltagewill also depend on the doping level of the layer 4 and the doping leveland depth of the region 5.

In an arrangement, the highly doped p-type regions 5 will have a dopingconcentration of at least 10¹⁶ cm⁻³, in a further arrangement at least10¹⁷ cm⁻³ or 10¹⁸ cm⁻³. In an arrangement, the doping concentration ofthe rest of the first layer 3 is at least a factor of 10 lower than thatfor the high field zones 5, in a further arrangement a factor of 100lower. The doping level of the region 3 may be less than 10¹⁶ cm⁻³ forexample.

FIG. 12 shows third and fourth steps in the fabrication sequence of FIG.11. In the present arrangement, the two APDs have been formed at thesame time, with an InP cap being deposited on a highly doped (n++) InPlayer. The highly doped InP layer has been formed on an InGaAs layerwhich has been deposited on an InP substrate, as discussed with regardto FIG. 11. Two detection regions and two guard rings, one surroundingeach detection region, are formed via p++ doping, as discussed withregard FIG. 11. The InP cap layer includes an avalanche region undereach detection region.

To create the connection between the cathodes of the APDs and the biastee, a region adjacent to one of the APDs is etched down to the InPsubstrate (or kept clear using a mask). A metal contact is depositedonto the InP substrate, to form the connection between the bias tee andthe APDs. This metal contact may be deposited at the same time as metalcontacts (anodes) for the two APDs are deposited onto the surface of thep++ doped regions. A dielectric is then deposited over the whole area,covering the APDs, including the metal contacts.

The metal contact may be a Chromium/Gold double layer where the highlydoped p-type regions are InP

FIG. 13 shows a fifth step in the fabrication sequence of FIGS. 11 and12. Vias are then formed within the dielectric, above each metalcontact. These may be formed by introducing holes in the dielectricabove each metal contact (via etching or masking) and depositing metalwithin the holes.

At the same time, a resistor thin film may be deposited adjacent to eachAPD to form the resistors that are connected to the outputs of the APDs.The lower layers for the resistors, capacitors and inductors may all bedeposited at this time. An upper layer of dielectric is then depositedover the whole region (including the vias and the resistor).

FIG. 14 shows a sixth step in the fabrication sequence of FIGS. 11-13.Vias are formed in the upper layer of dielectric above each via. Viasmay also be formed at the same time above each end of the resistor thinfilms, and in the capacitors and inductors in the circuit. The vias maybe formed in the same way as discussed earlier. Metal contacts aredeposited above the vias in the APDs which are then connected to therespective resistors to form the output of each APD. A connection isformed between the via connected to the InP substrate and the bias teeto allow the cathode of the APDs to be driven.

A metal connection is formed between the via above the metal contact inthe resistor and the adjacent via above the resistor thin film. A metalconnection may also be formed between from the via above the oppositeend of the thin film, leading to ground. The connections between thecircuit components may all be deposited at this point. These vias, metalcontacts and metal connections may be deposited at the same time.

FIG. 15 shows two final steps of the fabrication sequence of FIGS.11-14. The dielectric between the metal contacts in each APDs is etchedaway to expose the active areas of the APDs. Finally, an anti-reflectivecoating is deposited over the active areas. The anti-reflective coatingmay be deposited over the whole region, including the dielectric,provided that it does not cover any contact pads.

The material of the anti-reflective coating may depend on the wavelengthof light intended for the detector. For example, for an InP baseddetector, silicon nitride with a selected thickness may be used so thatthe reflection at the surface is minimal.

The region in each APD between the metal contacts and covered inanti-reflective coating is the detection region.

Each APD may be operated in Geiger mode. In Geiger mode operation, areverse voltage that exceeds the breakdown voltage is applied to theAPD. Light incident on the surface of the device on which theanti-reflective coating 9 is coated, i.e. at the side of the substrateon which the layers are fabricated, is absorbed and generates carriers.The light is absorbed in the region 2, generating carriers which driftto region 3 to multiply in the high electric field regions due to impactionisation. The high electric field across the interface between thehighly doped n-type layer 4 and the highly doped p-type regions 5 meansthat avalanche multiplication occurs in these regions when a voltageabove the breakdown voltage is applied across the junction. Thegenerated carriers are thus multiplied in the avalanche multiplicationregion. The resultant output signal for each detection region, V_(OUT),is measured at the corresponding anode contact 8. The detection regionscomprise the regions between the metal contacts 8 and covered with theanti-reflective coating 9, from the highly doped region 5 down to thehighly doped layer 4.

A time varying voltage is applied through the bias tee circuit. An ACvoltage component V_(AC) and a DC voltage component V_(DC) are combinedusing the bias-tee circuit. The bias tee circuit is connected to thecathode contact of the APD, i.e. metal contact 10. The bias voltageapplied to the APD thus comprises both a DC component and an ACcomponent. The bias voltage applied to the APD is above the breakdownvoltage at its highest values and below the breakdown voltage at itslowest values. When the bias voltage exceeds the breakdown voltage thedetector is gated “on”, when it is below the breakdown voltage thedetector is gated “off”.

In an arrangement, the output of the APD for each detection region ismeasured at a resistor which is connected to ground. Each anode contact8 is connected to a resistor. When a photon is incident on the detectionregion, an avalanche photocurrent is induced, which leads to a voltageacross the resistor corresponding to the output voltage, V_(out). Othercircuitry may be used to measure the electrical output of the detectionregions.

Conversely, as discussed with regard to FIG. 9, the bias voltage may bereversed and applied to the anode contacts, with the output voltagesbeing taken from each APD separately through the substrate 1. This wouldrequire an insulator to be formed in the substrate 1 between the twoAPDs.

FIG. 16 shows cross-sectional views of a capacitor and an inductor.

To form a capacitor, a bottom metal electrode is deposited on a firstdielectric layer. A second dielectric layer is deposited over the bottommetal electrode and a via is formed in the second dielectric layerconnecting to the bottom electrode. A top electrode is formed above thebottom electrode, on the second dielectric layer. The metal of the viaand the top electrode may be deposited simultaneously.

To form an inductor, a bottom spiral of metal may be formed on a bottomdielectric layer. A top dielectric layer may be formed over the bottomspiral. Top spiral of metal may be formed over the top dielectric layer,above the bottom dielectric layer. A via may be formed to connect thetop and bottom spirals. The metal of the via and the top spiral may bedeposited simultaneously.

FIG. 17 shows a method for forming a single photon detector according toan arrangement. The method details the steps discussed with regard toFIGS. 11-16.

Initially, the layers of the APDs are formed 1102 to produce thestructure shown in the top of FIG. 11. The corresponding layers for bothAPDs may be formed at the same time. Then the highly doped region andguard ring for each APD is formed 1104 to produce the structure shown atthe bottom of FIG. 11.

A region adjacent to the APDs is etched down to the substrate 1106 toform the structure shown in the top of FIG. 12. The anodes and cathodesare then deposited 1108. This may be performed in the same depositionstep. The anodes for each APD are deposited onto the highly doped regionof the APD. A single, joint cathode is deposited onto the substrate inthe etched region to form the structure shown on the bottom of FIG. 12.

A first dielectric layer is then deposited over the APDs and the anodesand cathodes 1110. Holes are etched in the first dielectric layer downto the anodes and cathodes 1112.

The metal for the vias, and for the bottom level of the resistors,capacitors and inductor (the components shown in FIG. 10) is thendeposited 1114. Again, this may be deposited in the same depositionstep. This forms the resistor thin film for each resistor, the bottomspiral in the inductor and the bottom electrode in each capacitor in thecircuit of FIG. 10, as well as filling the etched holes down to theanodes and cathodes.

A second dielectric layer is then deposited 1116 over the circuit (overthe first dielectric layer and the inductor, resistors, capacitors, viasand APDs). Holes are etched for vias in the second dielectric layer1118. This includes holes down to the vias in the first dielectriclayer, as well as vias down to the bottom layers of resistors,capacitors and inductors.

The metal for the vias, the circuit connections shown in FIG. 10 and thetop layers of the resistors, capacitors, and inductor are then deposited1120. This can be performed in the same deposition step. This forms topelectrodes of the capacitors and the top spiral of the inductor. Thiscompletes the connections down to the anodes, cathodes and resistors.This also forms the electrical links between the components.

Finally, the dielectric between the anodes of each APD is etched away toexpose the active areas (top structure of FIG. 15) and theanti-reflective material is deposited over the active areas (bottomstructure of FIG. 15) 1112.

A capping layer may be applied over one of the APDs to prevent it fromdetecting light. This ensures that this APD is the dummy APD.Alternatively, both APDs may be capable of detecting light; however,only one may be used at a time so that the other can act as the dummyAPD.

By implementing the bias tee and APDs on the same substrate, the size ofthe detector can be greatly reduced and the speed and accuracy of thedetector can be improved. In addition, by implementing two APDs, withone acting as a dummy APD, and forming them on the same chip, anyfabrication differences between the two APDs can be minimized. Thisallows the dummy APD to more effectively cancel out the capacitiveresponse of the active APD.

Whilst FIGS. 9-17 discuss various arrangements of components and methodsof forming said components, it will be appreciated that each componentmay be formed via alternative means and arranged in different formsprovided that they achieve their desired function. For instance,alternative materials for each APD may be used, with alternative oropposite doping properties, provided that the APDs are identical andeach is able to detect a single photon incident on its active region(subject to any blocking layer that may be placed above the dummy APD).In addition, whilst the method of FIG. 17 discusses depositing somelayers for different components at the same time, it will be appreciatedthat different deposition sequences may be utilised.

While certain arrangements have been described, the arrangements havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and devices describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made.

The invention claimed is:
 1. A photon detection system comprising: aphoton detection device comprising: a first input terminal for receivinga DC input voltage; a second input terminal for receiving an AC inputvoltage; a bias tee connected to the first and second input terminalsand configured to combine the AC and DC input voltages to form acombined voltage on an output of the bias tee; a first single photondetector connected to the output of the bias tee and configured toreceive the combined voltage from the bias tee, register a detectionsignal based on only a single photon being incident on the first singlephoton detector and output the detection signal indicative of thedetected photon; a first output terminal connected to an output of thefirst single photon detector for outputting the detection signal; asecond single photon detector connected to the output of the bias tee,in parallel to the first single photon detector, and configured toreceive the combined voltage from the bias tee and output an outputsignal; and a second output terminal connected to an output of thesecond single photon detector for outputting the output signal, whereinthe photon detection device is formed in a single integrated circuit;and a noise cancellation circuit configured to cancel a capacitiveresponse of the first single photon detector by comparing a part of thedetection signal with an earlier part of the detection signal to removenoise from the detection signal, wherein the earlier part of thedetection signal is phase shifted by 180 degrees.
 2. A photon detectionsystem according to claim 1 wherein the bias tee comprises: a firstinput line connected to the first input terminal, the first input linecomprising a low pass filter; and a second input line connected to thesecond input terminal, the second input line comprising a high passfilter.
 3. A photon detection system according to claim 2 wherein thelow pass filter comprises an inductor and a resistor and/or wherein thehigh pass filter comprises a capacitor.
 4. A photon detection systemaccording to claim 3 wherein the low pass filter further comprises asecond capacitor connected in parallel to the inductor and resistor. 5.A photon detection system according to claim 1, wherein the photondetection device further comprises a capacitive line for impedancematching connected between the output of the bias tee and ground, inparallel to the first single photon detector.
 6. A photon detectionsystem according to claim 1 wherein the first single photon detector isa single photon avalanche detector.